THIS INVENTION relates to a process for the production of dimers from an olefinic feedstock. More particularly, the invention relates to a process suitable for the production of a product having the general formula Rxe2x80x2Rxe2x80x3Cxe2x95x90CH2, in which Rxe2x80x2 and Rxe2x80x3 are alkyl groups, from an olefinic feedstock.
According to the invention there is provided a process for the production of dimers from an olefinic feedstock containing xcex1-olefins, the process comprising contacting the feedstock with a metallocene/ aluminoxane catalyst, thereby selectively to dimerize xcex1-olefins in the feedstock by means of a metallocene-catalyzed dimerization reaction, the feedstock being in the form of a Fischer-Tropsch-derived olefinic feedstock comprising a mixture of Fischer-Tropsch-derived hydrocarbons made up of at most 90% by mass of xcex1-olefins, at least 5% by mass of olefins, other than xcex1-olefins, selected from linear internal olefins, branched internal olefins, cyclic olefins, dienes, trienes and mixtures thereof, and at least 5% by mass of constituents, other than olefins, selected from paraffins, oxygenated hydrocarbons, aromatic hydrocarbons and mixtures thereof, the metallocene-catalyzed dimerization reaction taking place while the olefins which are dimerized form part of the mixture constituted by the feedstock.
By a Fischer-Tropsch-derived olefinic feedstock is meant an olefinic feedstock which is a product obtained by subjecting a synthesis gas comprising carbon monoxide and hydrogen to Fisher-Tropsch reaction conditions in the presence of a suitable Fischer-Tropsch catalyst, which catalyst may be iron-based, cobalt-based or iron/cobalt-based. In particular the Fischer-Tropsch-derived olefinic feedstock may be one which, after production thereof from the synthesis gas, has been subjected to no substantial further treatment, purification or processing thereof to remove unwanted constituents such as non-xcex1-olefins therefrom, other than cutting so that a suitable cut of the Fischer-Tropsch-derived product will typically be selected for use as the feedstock.
The feedstock may comprise 50-90% by mass of said xcex1-olefins, for example 60-80% by mass thereof; it may comprise 5-20% by mass of said olefins other than xcex1-olefins, for example 9-16% by mass thereof; and it may comprise 5-30% by mass of said constituents other than olefins, for example 13-22% by mass thereof.
By way of example, the feedstock may comprise 50% by mass of xcex1-olefins, 20% by mass olefins other than xcex1-olefins, and 30% by mass of constituents other than olefins. The metallocene/aluminoxane catalyst may have an aluminoxane component which is methylaluminoxane, and a metallocene component which is a compound of the general formula (Cp)2MY2 in which Cp represents a cyclopentadienyl group, M is a metal selected from zirconium, hafnium and titanium, and Y is selected from hydrogen radicals, halogen radicals (preferably chlorine radicals), alkyl groups (preferably methyl groups) and mixtures thereof. The metallocene component preferably comprises a single compound of said formula (Cp)2MY2, but it may instead comprise a mixture of several said compounds of formula (Cp)2MY2.
Still more particularly, the feedstock may comprise olefins other than xcex1-olefins which include linear and/or branched internal olefins, cyclic olefins, dienes and trienes, and constituents of the feedstock other than olefins may include paraffins, aromatics and small amounts of oxygenates. There may be an Al:M atomic ratio between aluminium in the aluminoxane component of the catalyst and the metal M in the metallocene component of the catalyst of 1:1-100:1, preferably 40:1-80:1, eg 60:1-70:1, As the Al:M atomic ratio increases, the degree of conversion and reaction rate increase, while the selectivity with regard to dimer production decreases, while a reducing Al:M atomic ratio reduces the degree of conversion and the reaction rate, and increases the selectivity for dimer production. An optimum or acceptable AI:M atomic ratio may accordingly be selected by routine experimentation, bearing practical and economic considerations in mind.
It is preferred to have the feedstock in a liquid state during the contacting of the catalyst with the feedstock, and the contacting may take place at a reaction temperature, and at a reaction pressure, both of which can vary within broad limits, the reaction time being determined by the period required to obtain a desired degree of conversion. Thus, reaction temperatures of xe2x88x9260xc2x0 C. to 280xc2x0 C., eg 20-120xc2x0 C. have been found to be suitable, and absolute reaction pressures of 1 atmosphere or less, up to 500 atmospheres or more, may be used. In a particular case, the feedstock may be in a liquid state during the contacting of the feedstock with the metallocene/aluminoxane catalyst, the contacting taking place at a reaction temperature of xe2x88x9260xc2x0 C. to 280xc2x0 C. and at an absolute reaction pressure of 1-500 atmospheres, usually at or slightly above 1 atmosphere; and in this case the reaction temperature if preferably 20-120xc2x0 C., the process being carried out under an inert atmosphere. Once again, routine experimentation may be used to establish optimum or acceptable reaction conditions with regard to temperature, pressure and reaction time.
In accordance with the process of the present invention the aluminoxane component of the catalyst, dissolved in an organic solvent (conveniently the organic solvent used in the preparation of the aluminoxane component) may be admixed with a well-stirred suspension of the metallocene component of the catalyst in an organic liquid which may be inert with regard to the metallocene/aluminoxane catalyst, or conveniently may be in the form of the feedstock used for the dimerization reaction. When the feedstock is used to suspend the metallocene component, the dimerization reaction proceeds in earnest as soon as the admixture thereof with the aluminoxane solution becomes substantially homogeneous. The dimerization is conveniently carried out under an inert atmosphere, eg an argon atmosphere, at atmospheric pressure.